Radar anti-jamming device



Oct. 27, 1959 R. w. RHYlNs RADAR ANTI-JWIN@ nEvIcE Filed Nov. 5, 1954Szusou INI/NTOR.

RICHARD w RHY//vs BY Wwf/ Afrox/vm mi W Kil@ QII 'Y 2,910,69l YPatented` oet.v 27, 1959 -ffice j RADAR ANTI-JAMMmG DEVICE Richard W.Rhyins, Harrison, NY., assgnor to General Precision'LaboratoryIncorporated, a corporation of New York Application Novemner 5, "1954,serial No. 467,131 12 C1aims."(C1.s43-f7) `'I'his invention relatestoradar instruments and more specifically to airborne radar instrumentscontaining circuits to protect them from malfunction caused by radiointerference. p j

This invention is specifically for application to airborne radarinstruments wherein the Doppler frequency component of the return orradar echo signal is employed in navigational instruments forascertaining the ground speed, drift.' angle, rate of climb or dive, andother purposes. l

The return or radar echo signal containing the Doppler information isgenerally a microwave radio frequency signal havinga frequency somewhathigher or lower than thatbf Vthe transmitted signal.V When thetransmitted signal orl one representative thereof is multiplied by theecho signal a product is obtained` having a frequency which is thedifference of the multiplicand and multiplier frequencies and whichcontains the Doppler information. Thisproduct actually contains, likethe radar echo signal, a band of frequencies having the characteristicsof noise with a Gaussian distribution of frequencies. For example, inone particular speed-measuring radar instruquency band somewhat widerthan the radar. intermedi ment the difference band of frequencies orDoppler spectrum has a central frequency which maybe anywhere in therange between 2,000 and 10,000 cycles persecond and the spectrum has abandwidth of 15% of its central frequency.

In order that'such radar systems may be made fully automatic they mustautomatically track or follow changes v in the quantity producing theDoppler information to produce an output signal continuouslyrepresentingthatA quantity. Since the magnitude of the quantity isrepref sentedbythecentral frequencyfof thee-Doppler information, such radar systemsinclude frequency .trackers asAv one of their most importantinstrumentalities. This frequency tracker includes a search circuitwhich on initialA loperation continuously scans or searches the Dopplerfrequency band for a Doppler return signal. When the signal is found,thefrequency tracker locks to it and thereafter stops scanning andfollows the Doppler signal.

Due to the nature of the radar return'spectrum and to the generallychanging nature of the terrain which may be flown over, the intensity ofthe echo undergoes wide and ripid fluctuations. rlhese fluctuations mayoften carry the echo intensity below its useful threshold value, whichnecessitates the use of a memory circuit to continue the functioning ofthe radar receiver-computer at the last known output duringbelow-threshold," periods. This memory circuit is switchedby acircuitwhich measures the ratio of the input signal intensity to theConstantin-v tensity level of the noise background.4 v l',

It is possible to cause such radar systems to malfunction bydeliberatelyor inadvertently subjecting them to microwave radioradiation from an external transmitter. Such a jamming transmitter wouldobviously have'to operate on the same-microwave frequency as vthe radarsystem and would'preferablybe modulated at a frequency corresponding, atleast momentarily, to the Doppler a transmitter should have a carrierfrequency equal tol the radar carrier frequency. "Since exactcorrespondence is very difncult to maintain at microwave frequencies, itis practically necessary to sweep the transmitter frequencies, that is,to frequency modulate the transmitter. In order to be effective thedeviation of this modulation should be such as to sweep the transmitterover a freate frequency pass band, because this pass bandfconstitutesVthe sharpest frequency discrimination encountered by a. received radarsignal. Likewise in order to cause serious interference withthe radaroperation the jamming transmitter frequency modulation must be at anaudio or video rate which coincideswith the audio or video frequencyof'the Doppler information being received byV the radar instrument atthe time. Thus the jamming frequency modulation must changerin itsmodulating frequencyV over a range approximately that of theJv Doppleraudio or video information. This frequency change may have a sawtoothform with a slope opposite to that of the radar scanning sawtooth.- Thejammingmodulating frequency may then coincide with the Doppler frequencymomentarily once in each cycle of the sawtooth jamming modulating form.`l

Such jamming may cause the radar system to malfunction by causing it togive a false output indication or no output indication. .The falseindication will be caused by impelling the radar receiver-computer torelease its lock on the radar return signal and to lock to the jammingVsignal. Tests have shown that this will occur only when the jammingsignal is relatively strong. By no output indication is meant that theradar system is shifted for an indefinitely long time to the memory modeof operation, thus depriving the radar system of the Doppler inmc.p.s.,and will partly sweepitif even further off` of malfunction will tend tooccur only when the Doppler return is so weak that it is very near theoperating threshold. The effect ofthe linterference is then to suppressthe Doppler return to below its threshold value and thus put thereceiver-computer on memory. However, as the receiver-computer is notlocked to `the interfering signal, the change should be more or lesstemporary.

Due to the nature of the effect of interference-during weak Dopplersignals, it is considered that Vthe ;radar system is suihcientlyprotected by its own signal-to-noise ratio (S/N) detection circuit. Theoperation of the instant anti-jamming device is therefore connedto thesituation when the Doppler return S/N is at high level, that is,forexample, S/N has a value between l0 and 45 db. Y

As a specific illustration, .let it be assumed that the radar microwavecentral frequency, as transmitted, is 8,000 mc.p.s., and that the rstprincipal bandpass limitation of the received signal is in theintermediate frequency amplifier, which has a central frequency of 30mc.p.s.`

and a pass bandwith of l mc.p.s.

The jamming transmitter might then bev constructed so that itV can betuned to a microwave frequency of 8,000 mc.p.s. It'is frequencymodulated with a defrz viation of i2 mc.p.s. so that its frequencyexcursions.

are from 7,998 to 8,002 mc.p.sl It thus will sweep the IF pass band of 1mc.p.s. even if off tune by 11/2 tune. The rate of frequency modulationis made to vary downward from 12 to l kc.p.s., then to change abruptlyto 12 kc.p.s. and repeat, the cycle taking 20 seconds. Thus the jammingtransmitter while sweeping the IF pass b'and,..srubjects the radarinstrument to an audio frequency varying over the complete Doppler audiofrequency range. 1 The jamming signal must therefore sweep the operatingDoppler central frequency and be in synchronism with it atleast onceeach 20-second cycle. If then the jamming signal is sufficiently strong,tests have shown that it lwill cause the radar instrument to leave theDoppler signal and lock to the jamming signal. At the end o=f thefrequency spectrum, if the lock should be broken, the radar frequencytracker will commence scanning. But since the radar scanning directionand the jamming modulation sweep direction are opposite, the jammingsignal will, during the ensuing cycle, again seize control of the radar.

. The defense against jamming is basedupon a difference in the outputsof the S/N detector under a jamming signal and under a strong radarsignal. The S/ N detector signal input is derived by heterodynefrequency conversion from the intermediate and audio amplifier output,

whichis highly gain-controlled by the intermediate frequency and audiofrequency automatic gain control circuits. The S/N detector contains afixed frequency channel having an output voltage representing the inputsignal strength plus the noise signal strength. The detector alsocontains a second channel operating at ak second fixed frequencyhaving-"an output voltage representing noise only. At Ahigh signallevels the Voltage output of the noise Achannel is much less than thatof the signal plus' noise channel.

When a jamming signal is received-during reception of a strong Dopplerreturn signal, the automaticv gain control reduces the gain and,although the jamming'signal after mixing is within the intermediatefrequency pass band, if its modulation is not at the eXact audioYfrequency of the Doppler return, the sharply tuned signal plus noisechannel will not perceive it and the voltage output of this channel willdrop with the automatic gain control gain reduction. Similarly, if theintermediate frequency jamming signal has modulation which whenheterodyned is not at the exact frequency of the sharply tuned noisedetector channel, that channel will not perceive it and its voltageoutput also will drop with the automatic gain control gain reduction.That is, jamming causes a concurrent drop in the outputs of the signalplus noise and noise channels.

This jamming action is not duplicated by a sudden increase in theDoppler return intensity. Such an increase also reducesfthe amplifiergain, and consequently reduces the noise channel output. However, theoutput of the signal plus noise channel goes up, not down. This is sobecause the automatic gain control action maintains the wideband signalplus noise voltage constant at the input to the S/N detector. Since thenoise portion of this signal is decreased by automatic gain controlaction throughout the wideband spectrum and at the output of the noisechannel, the signal plus noise voltage sampled at the Gaussian center ofthe wideband signal plus noise spectrum by the sharply tuned signal plusnoise channel must show a compensating increase. This increase may beshown to be proportional to the noise output decrease.

To recapitulate, an increased Doppler signal causes increased signalplus noise output with decreased noise output, lbut a jamming signalcauses both signal plus noise and noise outputs to drop.

The present invention applies differentiating circuits to the' outputs of the S/N detector circuit and by selectively polarized pulse circuitsdistinguishes between the two cases described above. The output iscaused to operate an alarm when the radar is being jammed.

One purpose of this invention is to provide an arrangement for providingan indication when the operation of a radar instrument of the describedclass is being interfered with, or jammed by radio signals.`

Another purpose of this invention is to provide an alarm indication whenthe operation of the radar instrument is deliberately interfered with byjamming its signals by the signals of a jamming transmitter.

A further understanding of this invention may be secured from thedetailed description, in which:

Figure l is the schematic Acircuit of apparatus embodying the inventionas applied to a Doppler radar instrument.

Figures 2, 3 and 4 graphically depict voltages at various points in thecircuit. l

Referring now to Fig. l, a microwave radar instrument is partlyrepresented by the antenna 11, microwave transmitter-receiver 12, mixer13, stable oscillator' 14, and intermediate and audio frequencyamplifiers 16, these amplifiers being automatically gain controlled. Theoutput at conductor 17 consists of an electrical potential having anaudio'spectrum of frequencies, and containing the Doppler frequencyinformation; The characterV of this signal is that of noise havingGaussian distribution about a central frequency between 2,000 and 10,000cycles, and having a spectrum width between half-power points of 15% ofthe central frequency.

A receiver-computer 18 finds the center of the Doppler spectrum andmeasures its frequency.Y In order to facilitate this operation theVDoppler signal in conductor 17 is frequency-transformed by anadjustable oscillator 19 and heterodyning mixer-modulator 21 to a fixedhigher frequency, 25 kc.p.s. being suitable.' This mixer-modulator iswide band, so that when the central Doppler frequency is transformed to25 kc.p.s. all energy at other frequencies within thevwide Dopplerspectrum, and including the entire constant noise background over the 2to 10 kc.p.s. band, is transformed to frequencies above and below. 25kc.p.s. This new spectrum is applied through isolating'resistor 22' tothe receiver-computer 1S. The latter controls the oscillator 19, Y bymeans Well known to those skilled in this art, to maintain thetransformed Doppler central frequency at exactly 25 kc.p.s., thisoperation constituting frequency tracking. In another mode of operation,when searching, the`25 kc.p.s. output of the heterodyne'frequencychanger Z1 is caused to represent a Doppler input frequency continuouslyscanned across -the band.

In order to detect and measure the signal-to-noise ratio of the signalapplied to the receiver-computer an S/N ratio detector is provided. Thisdetector consists of a signal-plus-noise amplifier 23 very sharply tunedto 25 kc.p.s., a noise amplifierVV 24 very Vsharply tuned to 21 kc.p.s.,a demodulator comprising diodes 26 and 27, and equal subtractingresistors 28 and 29. The output potential at median terminal 31represents the intensity of the Doppler signal above noise andtherefore,since the noise Vintensity is consideredl constant, it also represen-tsthe points lto which to connect equipment for carrying out the purposeof this invention. At these points are found the output voltages of thetwo S/ N detector channels, so that changes in these voltages representchanges inthe signalplus-noise and in noise, respectively. vSince thecathode 25 of `diode 26 is connected to terminal 33, the demodulatedenvelope of potential at that terminal is at a positive level. The anode30 of diode 27 isconnected to terminal 34, so that its signalv level hasan average negativev potential. These polarities, or thereversefpolarities,

aresuitable for the production of the normal VSN output' signal atterminal 31. However, for the purposes of this.

circuit arrangements remaining to be described are modified accordinglyso that in the presence of jamming a selected combination of voltagechangesenses at terminals 33 and 34 results in the output signal, of theanti-jamming device.

The two S/ N detector output potential 'signals are'first integrated toremove relatively high-frequency uctuations inherent in Doppler-derivedsignals secured from terrain reections. One integrating circuit consistsof resistor 36 and capacitor 37 connected between terminal33 and ground,and the othersimilar integrating circuit consists of resistor 38 andcapacitor 39 connected between terminal 34 and ground. A suitable timeconstant for these circuits is 0.3 second. Y

The signals are next differentiated since the device operates on changeof amplitude rather than on amplitude alone. Accordingly adifferentiating circuit consisting of capacitor 411 and resistor'42 isconnected between the median terminal 43 and ground, and anotherdiierentiating circuit consisting of capacitor 44 and resistor 46 isconnected between terminal 47` and ground. A suitable time constant forthese differentiating circuits is 3 seconds. These differentiatingcircuits are connected to high impedance loads, the terminal 48 beingconnected to the control grid 4'9 of a triode amplilier 51 and theterminal 52 being connected to the control grid``f534 of a triodeamplifier tube 54. Another triode amplifier tube 56 both amplies andinverts"'the signal output of amplier' tube 54.

The outputs are takenv from 'the anodes of amplier tubes 51 and 56 andare applied through 'blocking capacitors 57 and 58 .to a coincidencecircuit comprising pen-V tode 59. This tubetirs provided. with cathode,anode and screen resistors 61, 62 and 6 3, and has inputs applied to thefirst grid 64 and third grid 66; This tube is of a type, such as 6AS6,which is controlled by third grid potentials nearly or quite as well asby trst grid potentials. Outv put is taken from the cathode. Thecoincidencefoutput after amplification in triode 67 is usedtooperateapfrelay having a coil 68 connected in the anodejci'rcuit of triode 67and provided with locking contacts 69Vand alarmcontacts 71. Thelatterare connectedto lighta lamp 7,2

and the locking contacts 69 `are connected in series with The strongDoppler return intermediate frequency input signal, when constant exceptfor randomhigh fre'-v quency uctuations, is represented in Fig. 2 by thestraight line S. If a jamming signal be applied it may` be represented,at the input to the intermediate frequency amplitier, as a rising stepfunction J. e This increase of input intermediate frequency signalpower, acting through the intermediate frequency and 'audio frequencyautomatic gain controls, reduces the amplifier gain as depicted by' thegraph G. The reduction in gain is such as tomaintain the broadbandsignal plus noise signal constant at the input conductor 74, Fig.' 1, ofthe S/N detector. However, assuming that the audio frequencylof thejamming signal is not at the initial instant such as-to be hetero-Vdyned to either 25 lc.p.s. or 21 kc.p.s., bothof the S/N detectoroutputs will vdrop when thearnplier gainfdrops.

These drops are represented by a descending step graph 33', Fig. 2,since'the positive voltage level at terminali 33, Fig. 1, Vis reduced,and by a rising step graph 34', Fig. `2, becauseV the 'negative Avoltagelevel at 'terminal 34|,5Fig. vv1, is brought nearer to zero. The dashedline @represents Vzero potential.,` Y

Y The 'step potentials 33' and 34', representing conditions'aftterminalsl33 and 34, Fig.V l, are differentiated by the describeddifferentiating circuits to produce at the control grids 49 and53, Fig.1thegpulses'49' and 53', Fig. 3. 'These pulses are amplified, and sincethe noiseV is on' the order. of 10% of' the signal-plus-noise, the noisepulse is preferably amplified to a greaterfextent. Additionally ,one ofthe pulses must 'be inverted'so that their polarities will be thesame'atj'the coincidence input. Cousequently an odd number of'amplification stages in the signal-plus-noise channelis indicated bythe triode .51, Fig.` l; and an' even number ,of stages in the noisechannel is'indicated by the'triodes 54 and 56, so that the outputsfconsist of two positive pulses. Y

v "For proper operation of this circuit it is necessary thattheicoincidence circuit. operate only on time-coincident pulsesV of aselected polarity and having more than a certain minimum thresholdamplitude. The preferred coincidence circuit about (to be described hassuch inherent characteristics. However, when some other coincidentcircuit having marginal characteristics is employed it maybe preferableto clip and llimit the input signals,

either by biasing the amplifiers l51N, 54 and 56 or by insertingclipping circuits, `to obviate the possibility of operation Vofcoincidence circuit operation on only one ,ofV

thetwo input signals, or on input signals of wrong polarity.

64 of 'the coincidence pentode 59 are, both positive as depictedfbygraphs 66 and 64' respectively,1fFig. 4. 4The pentodet?j is sobiasedthat in the absence of pulses no angde current Hows and negativepulses, or a single posi'- tivepulse, likewiseA do not produce/anodecurrent ow."

produce'sa potential f drop in cathode resistor 61 and a.

positive pulse is derived therefrom. ,This positive pulseiscoupledthrough,capacitor v76to the control grid 77 Of'the amplierltube 67,A resulting in operation ofvits anode relay coil 68. Upon relayclosure contacts 69 apply locking potential to coil 68,*locking therelay closed. Contacts 71 lightlamp 72 which remains lit after the Vendof the pulse as a' signal'to the operator that a jamming signal hasbeen; perceived 4by the radar receiver. It `will'tbewnoted thatoperation ofthe falarmlamp 72 is not dependent uponf disturbance ofproper operation of the radar instrument, .but only upon the receptionofa jamming signal whichmayor may not have been powerful enough to seizecontrol'of the receiver-computer.

The spring-loaded'rr'esetI buttonl73, when pressed by the operator,opens the relay locking circuit, resetting the relay to its normalcondition'and restoring the alarm lamp 72 to its unlit condition.

Analysis 'of circuit operation for otheri'nput signal conditionsrshowsYthat sudden increases or decreases of Doppler return signals, or suddendecreasesin jamming sigincidencemeansemitting anoutput signal only uponthe time coincidenceof said two single-pulse signals and only when thesensesth'ereof are in selected directions, and

meansffor utilizing said output vsignal indicating thepreshepulseslresulting from the initiation of the jamming` signalasiapplied tothe thirdV grid 66 and therst grid` 7 ence of anextraneoussignal being received'b'y the radar instrument.

2. In a Doppler radar instrument containing a signalto-noi'se ratiodetector having two channel voltages, antijammin'g' apparatuscomprising, means forv integrating said two channel voltages to` formtwo output voltages containingy reduced irregularities representingDoppler return ter-rain' variations, means dilfer'entiating said twooutput voltages to form two single-pulse signals, coincidence' meanssensing the absolutesense of each of said I two single-pulse signals andemitting an output signal upon the' time coincidence of said two pulsesignals of selectedsenses, and means utilizing said output signalindicating the presence of a jamming signal in the received input ofsaid Doppler radar instrument. i

3. In a `Doppler rad/arV instrument containing an automatic frequencytracker and a signal-to-noise ratio'detectorhaving two channel voltages,jamming alarm app-aratus comprising', integrating means connected ltosaid signal-to-noise detector to form from said two channel voltages twooutput voltages containing reduced Doppler return terrain variations,means differentiating said two output voltages to form two single-pulsesignals of-senses uniquely representing 'the onset or radiointerference, a coincidence circuit identifying said senses representinginterference and emitting an output signal upon saidY coincidentidentification, and alarm means actuated by `said output signalindicating the presence of radio interference.

4. In a Doppler radar instrument containing an automatic frequencytracker and a signal-to-n'oise ratio detector having a signal-plus-noisechannel and a noise channel, jamming alarm apparatus comprising,integrating means connected -to said signal-plus-noise channel and saidnoise channel to form a signal-plus-noise output voltage and a noiseoutput voltage both containing reduced Doppler return terrainvariations, means differentiating said signal-plus-noise output voltageto form at the onset of a jamming interference sign-al a pulse havingpolarity representing reduction of the signal-plusn'oise output voltageand differentiating said noise output voltage to form at, the onset of.a jamming interference signal a pulse having polarity representingreduction of the noise output voltage, a coincidence circuit identifyingsaid pulse polarities occurring coincidentally only at the onset of ajamming interference signal, and alarm. means actuated, by the output ofsaid coincidencefc'ircuit indicating the presence of jamming`interference.

5. A radar anti-jamming device comprising`,. a. signalto-noise ratiodetector havingVv two channel voltages, means differentiating said twoVchannel. voltages to form two. single-pulse signals representative offchanges thereoco-r incidence'. means emitting an outputV signal. onlyupon4 the time coincidence of said two single-pulseV signals and onlywhen they have selected polarities, `and means for utilizingv saidoutput signal to `indicate the presence of an extraneous signal receivedby said radar.

6. A frequency-tracking Doppler radar instrument including ananti-jamming device comprising, a signal-tocomprising, a,signal-to-noise. ratio detector.` having asignal-plus-n'oise channel andanoise channel; 4integratingmeans connected to said signal-plus-noisechannel and' saidi noise channel] to; formya Vsignal-.plus-.noise outputvoltage and a noise output voltage both containing reduced vDopplerreturn terrain variations, means differentiating saidVs'ig'nal-ials-noise output voltage to form at the onset of fa jamminginterference signal a pulse having polarity representing reduction ofsign'al-plus-noise output voltage and differentiating said noise outputvoltage to form atY the onset of a jamming interference signal a .pulsehavingv polarity representing reduction of the noise output voltage, acoincidence circuit identifying said pulse polarities occurringcoincidentally only at the onset of a jamming interference signal, andalarm means actuated by the output of said coincidence circuitindicatingthe presence of jamming interference. v

8. Doppler frequency-tracking radar instrument providing a jammingalarmA comprising, a signal-to-noise ratio detector having asignal-plus-noise channel and a noise channel, integrating meansconnected to said signalplus-noise channel and. said noise channel tosmooth the voltages thereof reducing Doppler return terrain variations,means differentiating said smoothed signalplus-noise voltage to form asignal plus noise pulse at the onset of jamming interference signalhaving av selected first polarity representing reduction ofsignal-plus-noise voltage magnitude, means differentiating said smoothednoise voltage to form a noise pulse at the onset of a jamminginterference signal. having a selected second polarity representingreduction of noise voltage magnitude, a coincidence circuit connectedfor operation by said .several differentiating means only upon thecoincident occurrence of two pulses of said selected first and secondpolarities, and alarm means actuated by the output of said coincidencecircuit indicating the presence of jamming interference. i

A Doppler frequency-tracking radar in accordance with cl-aim 8 in whichamplifiers and inverters are interposed between said differentiatingmeans land said coincidence circuit to bringv said signal-plus-noise andnoise pulse signals to substantially the same amplitude and to the samepolarity.

10`- lA Dopplery radar jamming control comprising, a wideband amplifierprovided with automatic gain control means having asignal representativeof a reflected Doppler shift echo signal impressed thereon and having anoutput whose amplitude is controlled in accordance with the amplitude ofsignal noise frequency energy' occurring. within the band of fequenciespassed by said amplier, wide band 4frequency changing means having theoutput of said wide band amplifier impressed thereon fortransforming thecenter frequencyV of the Doppler shift echosig'nal to a fixed selectedfrequency andl having an output which includes saidV fined selectedfrequencyand a wide band of signal frequencies above and below saidselected frequency,- a sharply v tuned signal having signal frequenciesat saidl selected frequency,` a

sharply tunednoise amplifier connectedto the outputV of said frequencychanging means tuned toa frequency which differs from said selectedfrequency connected to the output of said' frequency changing meansl andtransmittinglronly thatl poition of the Voutput thereof havingsignal.frequenciesv at? the tuned frequency of said noise amplifier,means connected to the output of said signal plus nois'e. ampliierforVproducing a signal pulse` by theabrupt change in amplitudev thereof,means connected to4 theoutput ofY said noise amplier for producing asignal` pulse by' the' abrupt change in amplitude thereof,

and lindicator means operated onlybythe sinu'xltaneous occurrence ofsaid-first and second mentionedl signal pulses.rseiecferpelafines. Y

ll. ADo`ppler'radar jammingcontrol'comprising, a

wide baiidfamplienprovidedwith automatic gain con-- trlineanshavinga'fsignal#representativeof a reflected Doppler 'shift' echosig'nalimpressed ther'eo'n and having an 'outputwhose "amplitude iscontrolled'in accordanceVV with the amplitude of signal and noisefrequency energy occurring within the band of frequencies passed by saidamplifier, a wide band mixer having the output of said amplifierimpressed thereon, an oscillator having its output impressed on saidmixer, means for controlling the frequency of said oscillator inaccordance with the center frequency of the output of said mixer wherebythe center frequency of the wide band output thereof is maintained at aselected frequency, a sharply tuned signal plus noise amplifier tuned tothe center frequency of said mixer connected to the output thereof andtransmitting signals only at said center frequency, a sharply tunednoise amplier tuned to a frequency which departs from the centerfrequency of said mixer connected to the output thereof and transmittingsignals only at said departure frequency, means connected to the outputof vsaid signal plus noise amplifier for producing aiirst pulse signalon the occurrence of a change in output amplitude, means connected tothe output of said noise amplifier for producing a second pulse signalon the occurrence of a change in output amplitude, and means operatedonly by the simul-v taneous occurrence of firstrand second pulse signalsof selected polarities.

12. A Doppler radar jamming control comprising, a wide band amplifierprovided with automatic gain control means having a signalrepresentative of a reflected Doppler shift echo signal impressedthereon and having an output Whose amplitude is controlled in accordancel10 with the amplitude of signal and noise frequency energy occurringwithin the band of frequencies passed by said amplier, automaticallycontrolled wide band frequency changing means the output of which is aWide band of signal frequencies centered about a selected fixedfrequency having the output of said wide band amplifier impressedthereon, a sharply tuned signal plus noise amplifier tuned to saidselected frequency connected to the output of'said frequency changingmeans and transmitting signals only at said selected frequency, asharply tuned noise amplifier tuned to a frequency which vdeparts fromsaid selected frequency connected to the output of said frequencychanging means and transmitting signals only at said departurefrequency, differentiating means connected to the output of said signalplus noise amplifier producing therefrom a first pulse signal on theoccurrence of a change in output amplitude thereof, differentiatingmeans connected to the output of said noise amplifier producingtherefrom a second pulse signal on the occurrence of a change in outputamplitude thereof, and means operated by the simultaneous occurrence offirst n second pulse signals of selected polarities.

2,586,605 n Blumlein". Feb. 19, 1952

